Redirect all traffic regardless of port or protocol to temporary page

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http://serverfault.com – I have a server with multiple domains. Both http (apache) traffic and http-alt (tomcat) traffic. If/when I take the server down, I'd like to be able to redirect people to a temporary 'undergoing maintenance' page, regardless of their destination URL/port. How can this be achieved? Do I need tomcat running on the secondary server or is there a way to catch all and redirect via some method I'm not aware of? I've tried simply forcing all 80/8080 traffic to the secondary server, but depending on the URL it doesn't resolve, and it never resolves when trying to access :8080/whatever. Any ideas? (HowTos)